Complex-Formation Titrations

based upon reactions that yield complex ions or coordination compounds

a A + b T --In c Product Polyvalent Ligand Complex Ion / Coordination Compound Cation or Complexing Agent

remarkable growth in their analytical application is based upon a particular class of coordination compound called chelates

widely used for titrating cations

Complex-Formation Titrations

Uses of Coordination Compounds

1. Formation of Colored Substances can be used to indicate the presence of a specific

metal ion and/or determine its concentration in the sample

2. Chemical Masking masking agent decreases the concentration of a free

metal ion to a level where a particular interfering reaction will not occur

3. Titration of Metal Ions

Ligand donor

species

must have at least one pair of unshared electrons available for bond formation

A.Nature

1. Inorganic 2. Organic

H2O, NH3, Cl1- EDTA, NTA

B. Number of Electron Pair Donors

1. Unidentate 2. Polydentate or Multidentate

H2O, NH3, Cl1- EDTA, NTA

Complex-Formation Titrations

Coordination Number number of covalent bonds that a cation tends to form with electron donors

e.g. two, four, six

Chelate - produced when a metal ion coordinates with two (or more) donor groups of a single ligand to form a five or six- membered heterocyclic ring

Cu 2+ + 2 GLYCINE

Titrants : Complexing Agents

polydentate / multidentate

generally react more completely with cations and thus provide sharper end points

ordinarily react with metal ions in a single-step process

NTA Nitrilotriacetic acid

second most common complexing agent used for titrimetry

tetradentate ligand

Titrants : Complexing Agents

EDTA Ethylenediaminetetraacetic acid

free acid, H4Y and the dihydrate of the sodium salt, Na2H2Y

. 2 H2O, are the commercially available in reagent quality

forms very stable, 1:1complexes with most metal ions

its dilute solutions are very stable

it is sensitive to pH of the solution, thus it is somewhat selective in its reactivity with different metal ions

hexadentate ligand

1. Xylenol Orange

weak complexing agents

exhibit different colors in their complexed and uncomplexed form

one of the few indicators that can be used in acidic solutions

Indicators

2. Eriochrome Black T oldest and most

widely used complexation indicator

used exclusively in the pH range 7 to 11

oxidized slowly by dissolved oxygen, ascorbic acid is sometimes added to retard this reaction or solid EBT is added directly to the sample prior to titration

Indicators

3. Calmagite 4. Arsenazo I

structure is very similar to EBT

more stable than EBT in aqueous solution

excellent indicator for EDTA titrations of the rare earths

unlike EBT and Calmagite, not blocked by small amounts of copper or iron (III) during calcium and magnesium titrations

Indicators

Analytes

virtually every metal cation with the exception of the alkali metal ions

determinations of analyte or mixture of analytes involve one or a combination of the titration methods

Applications

1. Direct Titration Method

Determination of Water Hardness

expressed in terms of the concentration of calcium carbonate that is equivalent to the total concentration of all the multivalent cations in the sample

hard water precipitates calcium carbonate upon being heated, which then clog boilers and pipes

calculated as ppm CaCO3

Applications

2. Back-Titration Method

cations that cannot be titrated directly but form very stable EDTA complexes are good candidates for back-titration

3. Indirect / Replacement Titration Method

unavailability of a suitable indicator may also be overcome by replacing the analyte with an acceptable substitute

Exercise 14

An antacid tablet weighing 15.476-g was dissolved in acid and diluted to 500.0-mL.

A 25.00-mL aliquot of the solution was made sufficiently basic to precipitate the Al as Al(OH)3. The remaining Mg required 16.49-mL of 1.043 x 10 -2 M EDTA for titration.

A second 25.00-mL aliquot was withdrawn from the 500-mL flask and treated with 50.00-mL of the EDTA. This solution was made basic and the excess EDTA back titrated with 11.73-mL of 5.594 x 10 -3 M MgCl2. Calculate the percentage of both Mg and Al in the sample.

Molar Masses: Al = 26.98 Mg = 24.31

Exercise 14

An antacid tablet weighing 15.476-g was dissolved in acid and diluted to 500.0-mL.

A 10.00-mL aliquot of the solution was made sufficiently basic to precipitate the Al as Al(OH)3. The remaining Mg required 6.596-mL of 1.043 x 10 -2 M EDTA for titration.

A second 25.00-mL aliquot was withdrawn from the 500-mL flask and treated with 50.00-mL of the EDTA. This solution was made basic and the excess EDTA back titrated with 11.73-mL of 5.594 x 10 -3 M MgCl2. Calculate the percentage of both Mg and Al in the sample.

Molar Masses: Al = 26.98 Mg = 24.31

Exercise 14

A 0.4085-g sample containing lead, magnesium and zinc was dissolved and treated with cyanide to complex and mask the zinc:

Zn 2+ + 4 CN 1- Zn(CN)4 2-

Titration of the lead and magnesium required 42.22-mL of 0.02064-M EDTA. The lead was next mask with BAL (2,3-dimercaptopropanol), and the released EDTA was titrated with 19.35-mL of a 0.007657-M magnesium solution. Finally, formaldehyde was introduced to demask the zinc:

Zn(CN)4 2- + 4 HCHO + 4 H2O Zn 2+ + 4 HOCH2CN + 4 OH 1- which was titrated with 28.63-mL of 0.02064-M EDTA. Calculate the percentages of the three metals in the sample.

Molar Masses: Zn = 65.37 Pb = 207.2 Mg = 24.31